As is known in the art, the increase of the field strength of newer MR imaging machines (such as the Siemens Trio MRI) provides the opportunity for much improved image resolution and quality. A challenge with these systems is that the increased magnetic strength has resulted in magnifying B0 effects (a magnetic loading distortion effect). B0 effects present themselves as a localized distortion in the volume. B0 distortion can occur anywhere within the volume, but is most pronounced in the regions on the periphery of the iso-center of the volume. Much effort has been made to correct for these effects in the design of MRI installations [see J. Sled, G. Pike, Correction for B0 and B1 variations in quantitative measurements using MRI, Magnetic Resonance in Medicine, Vol 43, No. 4, 2000, p. 589-5931].
As is also known, whole body scans of a patient can take up 3-5 individual volume acquisitions which then must be composed together in order to capture a complete scan of an individual. Since B0 effects are most evident in the leading or trailing edges of a MR volume, having sufficient volume overlap and discarding the leading or trailing regions is a possible solution. However, these kinds of solutions will inevitably result in the need for more volumes, and the more volumes that are required for an individual, the more time individual subject workflow takes which can cut into the efficiency of a MR installation. Also, there is no certainly where B0 effects might occur in a volume. This makes narrowing the field of view of a volume an expensive and imprecise solution.
The other alternative to excising regions of a volume affected by B0 is to attempt to correct for it. Modeling the B0 effects in an image can allow for the use of image transformation filters in order to diminish or remove the distortions in a particular image. This would allow for a greater percentage of a volume acquisition to be made useable. One way is to create a volume distortion field which maps the distortion present in a particular volume. Modeling distortion requires at least measurement of the distortion in particular points or reference features in the image. If sufficient points or reference features are known, then a volume distortion field could be generated by fitting these points to a 3D fitting function. Image rectification of distortion is a well studied [see B. Zitova, J. Flusser, Image Registration Methods: A Survey, Image and Vision Computing, Volume 21, Number 11, October 2003, pp. 977-1000(24)].
Phantoms and feedback sensors are used to calibrate a MRI magnet and detect for distortion. B0 distortion is not solvable using calibration techniques either with or without a phantom since the mass/density distribution is idiosyncratic to a particular subject and cannot be exactly reproduced. The differences between patient and phantom at separate time instances will degrade any calibration technique that could be provided.
The method according to the present invention is to use image clues (e.g., reference identifiable characteristics) within a volume image of a patient itself, or a separate volume image of the patent taken during the same session. By only considering the identifiable characteristics within the same session, inter-temporal variation in a subject, MRI magnetic field variation, or unaccounted workflow variation can be avoided. These reference identifiable characteristics provide a “ground truth reference” image and are used to calculate distortion on a high resolution and high contrast volume image of the patient. The calculated distortion is used to correct B0 distortion in the high resolution and high contrast volume image.
Three methods are suggested for developing these reference points or reference identifiable characteristics: 1. Use of a whole body scout localization volume as the “ground truth reference” (i.e., the reference identifiable characteristics); 2. Use of a co-localized volume less sensitive to B0 effects as the “ground truth reference identifiable characteristics”; and 3. Use of tagged reference object markers as the “ground truth identifiable characteristics”.
In accordance with the present invention, a method is provided for reducing B0 inhomogeneous effects in magnetic resonance imaging (MRI). The method includes: obtaining a high-resolution volumetric MR image of the patient, such high-resolution volumetric MR image having B0 inhomogeneous effects; calculating distortion within obtained the high-resolution volumetric MR image of the patient; and correcting the B0 inhomogeneous effects in the obtained high-resolution volumetric image using the calculated distortion.
In one embodiment, the distortion calculation includes obtaining a secondary reference volumetric MR image of the patient having reference identifiable characteristics; and using the reference identifiable characteristics to calculate distortion in the high-resolution volumetric MR.
In one embodiment, the calculation distortion comprises obtaining identifiable characteristics from the obtained high-resolution volumetric MR image and the correcting comprises performing image rectification on the obtained high-resolution volumetric MR image of the patient
In one embodiment, the method includes: obtaining a reference volumetric MR image of a patient; extracting a first set of identifiable characteristics on the reference volumetric MR image; extracting a second set of features on the high-resolution volumetric MR image; and wherein the calculating comprises registering the reference volumetric MR image and high-resolution volumetric MR image onto a common reference coordinate system and using the extracted first set of identifiable characteristics and the second set of identifiable characteristics.
In one embodiment, the high-resolution image comprises a plurality of images, each one of the images being of a different portion of a region of the patient.
In one embodiment, the reference volumetric MR image is of the region of the patient.
In one embodiment, the reference volumetric MR image comprises a plurality of reference images, each one of the reference images being of a corresponding different portion of the region of the patient.
In one embodiment, the high resolution and the reference volumetric MR images are obtained with different protocols.
In one embodiment, the high-resolution volumetric MR image is obtained with a reference template placed on the patient, such template having MR identifiable markers, such markers providing the first set of identifiable characteristics.
In one embodiment, the method includes using a secondary image volume of the patent taken during the same session to assist in rectification of a primary image volume. This secondary volume is selected to maximize resistance to B0 effects and speed of acquisition. The primary image volume is selected for the characteristics of image resolution and image contrast that are critical to diagnosis. The secondary image volume is used to rectify to primary image volume, using a process of registering volumes to allow for generation of distortion field.
In one embodiment, the method includes: obtaining a high-resolution volumetric MR image of the patient, such high-resolution volumetric MR image having B0 inhomogeneous effects; registering the high-resolution image with a secondary image volume to obtain a resultant deformation field; and using the obtained resultant deformation field and image rectification on the obtained high-resolution volumetric MR image primary image volume to correct the B0 inhomogeneous effects in the obtained high-resolution volumetric image.
In one embodiment, the method includes: obtaining a secondary reference volumetric MR image of a patient; and registering the reference volumetric MR image and high-resolution volumetric MR image to corrected the B0 inhomogeneous effects in the obtained high-resolution volumetric image.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.